THE EFFECT OF GLOBAL CHANGE ON ST LUCIA, AFRICA'S LARGEST COASTAL LAGOON

St Lucia is Africa’s largest coastal lagoon and separated from the Indian Ocean by the highest vegetated coastal dunes in the world. Its present large surface (350 km2) and small depth (on average 90 cm) makes it sensitive to climate changes. Small changes in sea level have a major effect on the surface area and ecology.

During the Eemian Interglacial the estuary was a marine bay. The high coastal dune system was probably not fully developed at that time. During the lowering of the sea level in the Early Weichselian the climate became drier. The basin probably dried up and for a period became a river plain.

Pollen and the occurrence of peat indicate a short and rather moist episode around 23 000 BP. This might have been triggered by the high solar radiation in the southern hemisphere at that time. Fossil soil profiles along the shores of St Lucia are related to the same climate event.

Peat and ancient soil profiles are overlain by sand of up to several metres thickness. The sand contains marine diatoms and was probably originally deposited in the Eemian marine bay. The climate during the Last Termination was dry. The marine sand was eroded by wind from the now dry bottom of St Lucia and deposited over the present lowland coastal plain and dune system.

Due to the high surface area relative to the depth, St Lucia goes through major salinity cycles. It is not obvious how a future sea-level rise will influence upon the estuary conditions. There will be a higher inflow of seawater, but a wetter condition may on the other hand result in a larger outflow of freshwater. We can, however, safely conclude that the lake will not return to an Eemian condition. The high coastal dunes will now form an efficient barrier, and in contrast to the conditions during the Eemian these dunes will separate St Lucia from the Indian Ocean.